FIG. 1 shows a conventional LED driver 10 configured with a boost converter for LED Lighting, which includes a controller 11, an inductor L and an output capacitor Cout. The controller 11 is a semiconductor chip, the inductor L is coupled between a power supply VIN and a phase pin LX of the controller 11, and the output capacitor Cout is coupled to an output pin OUT of the controller 11. In the controller 11, an n-type metal-oxide-semiconductor (NMOS) transistor 22 serving as a power switch is coupled between the phase pin LX and a ground node GND, and a p-type metal-oxide-semiconductor (PMOS) transistor 24 serving as a power switch is coupled between the phase pin LX and the output pin OUT. The inductor L, the NMOS transistor 22 and the PMOS transistor 24 are so configured to be a boost converter, and a logic circuit 20 provides control signals S4 and S5 to switch the power switches 22 and 24 to produce an output voltage which is higher than the turn-on voltage VF of a LED 28 for lighting the LED 28. A current sensor 18 monitors the inductor current IL to produce a current sense signal S1, a slope compensator 12 compensates the current sense signal S1 to produce a signal S2, a current source 26 is coupled to a cathode of the LED 28 by a feedback pin FB of the controller 11 for sinking a constant current ICS, an operational amplifier 16 detects the voltage on the feedback pin FB to produce a signal S3, and a comparator 14 compares the signals S2 and S3 to determine its output provided for the logical circuit 20 to produce the control signals S4 and S5. The LED driver 10 is advantageous in having high efficiency when the input voltage VIN is lower than the forward voltage VF, and is disadvantageous in that the controller 11 requires larger die area because the boost topology needs a larger PMOS transistor 24 to act as a power switch for sourcing a high LED current ID. If it is to operate in larger LED current ID, it needs larger PMOS 24 and thereby more die area.
Therefore, it is desired a high efficiency LED driver which can be implemented with a small die area.